1. Field of Invention
This invention relates to the removal and filtration of heavy metals from an aqueous solution.
2. Background of Invention
Removal of heavy metals has several applications, including improved analyses, through concentration of metals, economy, remediation, and general concern for eliminating toxic substances from environmental samples or preventing their introduction to the environment. Concentration of metals from dilute aqueous samples would be economically sound for disposal purposes.
In the typical extraction process of a metal from an aqueous solution containing the metal values, the aqueous solution containing the metal values is contacted by a solution of a water insoluble extractant capable of extracting the desired metal, in a water immiscible hydrocarbon solvent. After contact for a sufficient time to extract at least a portion of the metal values, the hydrocarbon solvent phase, now loaded or containing the extracted metal values, is separated from the aqueous solution phase from which the metal values have been extracted, due to the immiscibility of the organic and aqueous phases. The loaded organic phase is then typically contacted with an aqueous stripping solution thereby forming two phases again, (a) an aqueous strip phase, now containing metal values stripped from the organic extractant phase, and (b) an organic phase from which the metal values have been stripped. Again the organic and aqueous phases are separated due to immiscibility of the phases. The metal is then recovered from the metal loaded aqueous strip phase, by conventional means, such as electrowinning, precipitation or other means suitable to the particular metal, generally electrowinning being the preferred recovery means. Both acid and ammoniacal aqueous solutions have been employed as stripping solutions in the past, one commercial process in the recovery of nickel employing an ammoniacal aqueous stripping solution. The organic extractant employed commercially in extraction of metals such as copper, nickel and zinc are the phenolic oxime extractants. In the process, particularly with aldoxime extractants, it is often desirable to include in the organic extractant phase an equilibrium modifier, to provide for the most efficient extraction and “net transfer” of the metal being recovered. In the process there is a transfer of metal in the extraction stage from the aqueous feed solution to the organic extractant phase, followed by a second metals transfer from the organic phase to the aqueous strip solution phase, the two metal transfers representing the “net metal transfer” of the process. Effectively “net transfer” can be determined by observing the difference between the extraction isotherms and the strip points. Typically equilibrium modifiers employed with the phenolic oxime extractants in the process have generally been various alcohols and esters. However, this process results in significant amounts of ammonia required for loading along with metal values into the organic phase. The ammonia must then be removed from the organic phase, with a high cost of ammonia, system costs and acid costs required to neutralize the ammonia carried in the organic phase by a scrub section.
U.S. Pat. No. 5,728,854 describes a method for separating iron from nickel and/or cadmium contained in a battery waste where the first step in which the spent batteries are crushed and calcined. The calcined pieces are mixed with an acetic acid aqueous solution before acetic acid and water are removed by evaporation or distillation so as to produce a residue containing metallic acetates. Water, the residue and an oxidant are mixed such that Fe+ and Fe++ acetates are converted into a basic ferric acetate, Fe(CH3COO)2OH, which is insoluble in water and is recovered by filtration.
Supported coordinating agents are also used for metal-ion removal. The method provides favorable equilibria for removal, together with the co-advantages of convenience and ease of separation. Supported coordinating agents afford the possibility of design of ligands for specific applications, both in the selection of the ligands to be used for given types of metals (calcophiles, with an affinity for sulfur, or lithophiles with an affinity for oxygen donors, etc). Supports include several polymeric systems which are available. Polystyrene impregnated with β-diphenylglyoxime is a selective reagent for palladium. Treated foam is used as a support for coordinating agents. Ion-exhange resins are used as supports for those coordinating agents that can be derivatized or converted to ions. Attachment of the ligand to the substract through derivatization is effective, using 3-chloropropyltrimethoxysilane as a means of attaching polyamines to silica gel. Such derivatization, however, means an additional step and at an additional cost.
Azeotropic distillation has been used for solvent removal, as described in on U.S. Pat. No. 5,807,506, which describes a process for the preparation of conductive polymers which comprising (a) mixing at least one monomer with at least one conductive component, solvent, at least one polymerization initiator, and an optional chain transfer component; (b) effecting solution polymerization by heating; (c) removing the solvent by azeotropic distillation in an aqueous phase to generate a mixture of polymer and conductive component; (d) drying and grinding the resulting mixture; thereafter dissolving the product resulting in at least one monomer, at least one initiator, and at least one crosslinking component, and an optional chain transfer agent to form an organic phase; (e) mixing said organic phase with a second aqueous phase comprised of water, stabilizer, and an alkali halide; (f) polymerizing the resulting suspension by heating; and (g) subsequently optionally washing and drying the polymeric product, and which product is comprised of polymer and conductive component dispersed therein. However, this method, although employs the azeotropic distillation to remove the solvent, is not applicable in removing metals from aqueous solutions.
U.S. Pat. No. 5,772,909 describes separation of vanillin from second organic chemicals by azeotropic distillation using as an effective azeotropic distillation agent, dibenzyl ether. Vanillin is difficult to separate from second organic chemicals produced therewith, such as parahydroxy-benzaldehyde by conventional distillation or rectification because of the proximity of their boiling points. This reference, although providing an azeotropic distillation method to remove agents from aqueous solutions, does not address the removal of heavy metals.
U.S. Pat. No. 5,597,476 describes catalytic cracked naphtha, desulfurized with minimum loss of olefins and octane. The naphtha is fed to a first distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a first distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds. The bottoms are subjected to hydrodesulfurization in a second distillation column reactor where the sulfur compounds are converted to H.sub.2 S and removed. The lighter fraction containing most of the olefins is thus not subjected to the harsher hydrogenation conditions of the second reactor. However, this reference also does not address the removal of metals from aqueous solutions with the use of mercaptans attached to a substrate.
Thus, what is needed is an improved method of removing heavy metals from aqueous solutions which is inexpensive, reliable and simple, along with applicability to variety of substrates. Nothing in the prior art including the aforementioned references implicitly or explicitly discloses the present invention covering the removal of metals, or heavy metals by mercaptans attached to silical gel by azeotropic distillation. Furthermore, nothing in the prior art discloses the products produced by such methods.